The LCD (Liquid Crystal Display) possesses many advantages of being ultra thin, power saved and radiation free. It has been widely utilized in, such as LCD TVs, mobile phones, PDAs, digital cameras, laptop screens or notebook screens.
For the TFT-LCD panels in the present main market, three types can be illustrated, which respectively are Twisted Nematic/Super Twisted Nematic (TN/STN), In-Plane Switching (IPS) and Vertical Alignment (VA). The VA liquid crystal display possesses extremely high contrast than the liquid crystal displays of other types, which can reach up to 4000-8000 in general. It has very wide application in large scale display, such as television or etc.
The reason why the VA liquid crystal display possesses extremely high contrast is that the liquid crystal molecules are vertically aligned to the substrate surface, and no phase difference exists, and light leakage is very small at the dark state without applying electricity. The lower the brightness at the dark state can be, the higher the contrast is. As shown in FIG. 1, for vertically aligning the liquid crystal molecules in the VA liquid crystal display to the substrate surface, the liquid crystal molecules 300 require vertical alignment process. The most common method is to coat the alignment agent in specific districts at the inner surfaces of the upper substrate 100 and the lower substrate 200, and the alignment agent generally contains a mass of chemical solvents, N-methylpyrrolidone (NMP) and polymer material (Polyimide, PI). Then, the substrate is baked for a long time under high temperature (200 degree above in general) to cure the solvent in the alignment agent, and thus, to form the PI alignment layer 310 on the surfaces of the glass substrates.
For earning better wide view angle property for the VA liquid crystal display, the multi-domain VA (MVA) technology is commonly utilized, which is to divide a sub pixel into many districts and drive the liquid crystals in respective districts to lie down toward different directions as applying voltage. Thus, the watch results from respective directions can be equal. There are many ways to realize MVA technology. Please refer to FIG. 2 and FIG. 3. One is to form a pozidriv slit pattern on the Indium Tin Oxide pixel electrode 800 at one side, and with the special ITO pixel electrode pattern, the tilt electric field can induce the liquid crystal molecules 300 to lay down toward different directions. FIG. 2 is a top view diagram of one side of a lower substrate 200 in an MVA liquid crystal display according to prior art, wherein 210 and 220 respectively are scan line and data line. FIG. 3 is a sectional diagram of an MVA liquid crystal display according to prior art, wherein 700 is a passivation layer of which the material is silicon nitride (SiNx). The MVA liquid crystal display has advantages of simple process, high contrast, high aperture ratio and short response time.
However, for realizing the VA alignment, the vertical alignment agent has to be coated on the substrate surface and high temperature baking process has to implement. Because the alignment agent contains a large amount of NMP solvent, Thus, the alignment layer formation process is a high energy consumptive, extremely non-environmental protective and human body harmful procedure; besides, due to the problems of non-uniformity, lack coating, non-stick and foreign matter of the alignment layer, the yield lost of the production can occur which results in source waste and increase of the production cost.
In consideration of the above factors, there is a need to develop the VA liquid crystal display in which the PI alignment layer is not needed. As shown in FIG. 4, by changing the recipe of the liquid crystal of adding substance of a sort of auxiliary alignment agent, make the liquid crystal molecules 300 to be vertically aligned to the substrate surface of the liquid crystal display under condition without alignment layers. Such kind of auxiliary alignment agent does not need other solvent but can be directly dissolved in the liquid crystal and used with the liquid crystal together. The previous coating equipment and high temperature baking equipment of the alignment agent can be saved. Without the extra solvent, it is cleaner, more environmental protective and energy saved. The function mechanism of the auxiliary alignment agent 301 is, one end of molecules of the auxiliary alignment agent possesses special affinity with inorganic material of the substrate surface, such as silicon nitride constructing the passivation layer 700 or ITO constructing the pixel electrode 800, and can attach to the surfaces of the substrates, and the other end of molecules of the auxiliary alignment agent 301 has extremely strong acting force with the liquid crystal molecules 300, and thus, the liquid crystal molecules 300 can be induced to be vertical aligned with the surfaces of the substrates. Ultimately, the effect shown in FIG. 4 can be achieved.
Although the small molecule auxiliary alignment agent can realize the effect of vertical alignment, the aligning force and the anchoring energy of most of the small molecule auxiliary alignment agents are weak, and result in issues of poor uniformity in surface and low long-term reliability. Thus, there is a need to improve such kind of liquid crystal display of achieving vertical alignment without PI to enhance the aligning force and the anchoring energy to the liquid crystal molecules.